Hydrothermal Synthesis of Heterostructured g-C3N4/Ag–TiO2 Nanocomposites for Enhanced Photocatalytic Degradation of Organic Pollutants

In this study, heterostructured g-C3N4/Ag–TiO2 nanocomposites were successfully fabricated using an easily accessible hydrothermal route. Various analytical tools were employed to investigate the surface morphology, crystal structure, specific surface area, and optical properties of as-synthesized samples. XRD and TEM characterization results provided evidence of the successful fabrication of the ternary g-C3N4/Ag–TiO2 heterostructured nanocomposite. The heterostructured g-C3N4/Ag–TiO2 nanocomposite exhibited the best degradation efficiency of 98.04% against rhodamine B (RhB) within 180 min under visible LED light irradiation. The g-C3N4/Ag–TiO2 nanocomposite exhibited an apparent reaction rate constant 13.16, 4.7, and 1.33 times higher than that of TiO2, Ag–TiO2, and g-C3N4, respectively. The g-C3N4/Ag–TiO2 ternary composite demonstrated higher photocatalytic activity than pristine TiO2 and binary Ag–TiO2 photocatalysts for the degradation of RhB under visible LED light irradiation. The improved photocatalytic performance of the g-C3N4/Ag–TiO2 nanocomposite can be attributed to the formation of an excellent heterostructure between TiO2 and g-C3N4 as well as the incorporation of Ag nanoparticles, which promoted efficient charge carrier separation and transfer and suppressed the rate of recombination. Therefore, this study presents the development of heterostructured g-C3N4/Ag–TiO2 nanocomposites that exhibit excellent photocatalytic performance for the efficient degradation of harmful organic pollutants in wastewater, making them promising candidates for environmental remediation.


Introduction
Nowadays, organic contaminants have become major sources of water pollution worldwide due to population growth and rapid agricultural and industrial development [1]. Modern commercial dyes are a major category of organic pollutants known for their robust structural and color stability owing to their high degree of aromaticity and extensively conjugated chromophores. With widespread use in diverse industries, such as food, cosmetics, leather, plastics, printing, and textiles, these dyes pose a health risk to humans suitable energy bandgap positions, can create a heterojunction that further boosts the photocatalytic efficiency of TiO 2 [5]. In recent years, the coupling of TiO 2 and g-C 3 N 4 has gained considerable attention owing to their highly compatible band positions [21][22][23][24][25][26]. A heterojunction formed between TiO 2 and g-C 3 N 4 reduces the recombination rate of charge carriers and improves photocatalytic activity [1]. Therefore, g-C 3 N 4 is considered to be the best candidate for coupling with TiO 2 to construct heterojunctions because of its suitable band position, chemical stability, and low cost [27]. Graphitic carbon nitride (g-C 3 N 4 ) is an n-type polymeric semiconductor that has been extensively studied owing to its potential use in the elimination of organic pollutants. It is cost-effective, non-toxic, eco-friendly, easy to synthesize, and has a mid-energy bandgap (2.7 eV) as well as strong physicochemical stability [6]. Despite its many advantages, bulk g-C 3 N 4 exhibits weak photocatalytic performance owing to factors such as fast recombination of photoexcited charge carriers, limited visible light absorption range, and low specific surface area [28]. Researchers have found that modifying the photocatalytic properties of g-C 3 N 4 is as important as modifying TiO 2 , and the synergistic effect of Ag deposition on TiO 2 and coupling with g-C 3 N 4 can lead to enhanced photocatalytic activity of the ternary composites [29][30][31][32][33][34][35][36]. In particular, heterostructured g-C 3 N 4 /Ag-TiO 2 nanocomposites have emerged as a promising area of research in the field of photocatalytic degradation of organic pollutants and have been extensively studied due to their remarkable visible-light-driven photocatalytic performance. Therefore, many strategies have been used to fabricate heterostructured g-C 3 N 4 /Ag-TiO 2 nanocomposites, including the microwave-assisted approach [37], freeze-drying route [9], hydrothermal method [36,38], calcination [39,40], physical mixing-calcination method [41], physical mixing [42,43], chemical reduction methods, and so on [11].
Most recent research findings on g-C 3 N 4 /Ag-TiO 2 nanocomposites have used xenon lamps with cutoff filters [11,14,15,17,[42][43][44] as visible light sources. However, they have various drawbacks, such as being hazardous, having a short lifespan, being overheated quickly, being expensive, and posing difficulties in handling [6]. However, LED lamps are non-toxic and energy-efficient alternatives to conventional lamps because they convert more energy into light and do not generate excessive heat. Furthermore, LED lamps are cost-effective, durable, and emit only the wavelengths required to save energy [45]. As part of green and sustainable chemistry, this study used an energy-saving 50 W LED lamp to measure the photocatalytic degradation efficiency of the as-synthesized samples.
In this study, a heterostructured g-C 3 N 4 /Ag-TiO 2 nanocomposite was successfully fabricated using an easily accessible hydrothermal technique, followed by calcination treatment. First, a simple sol-gel synthesis route was used to prepare pristine TiO 2 and 3 mol% Ag-doped TiO 2 nanoparticles. Second, g-C 3 N 4 nanosheets were synthesized using the single-step calcination of a mixture of urea and thiourea at 600 • C in an air medium. Finally, a facile one-step hydrothermal technique and calcination were used to prepare a g-C 3 N 4 /Ag-TiO 2 nanocomposite using a desired weight ratio of 3:1 from the fabricated g-C 3 N 4 nanosheet to 3 mol% Ag-TiO 2 nanoparticles. The structural, morphological, and optical properties and specific surface area of the prepared materials were characterized by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) nitrogen adsorption/desorption technique (BET), photoluminescence (PL), and UV/vis diffuse reflectance spectroscopy (DRS). In addition, the photocatalytic performance of the prepared pristine TiO 2 , binary Ag-TiO 2 nanoparticles, g-C 3 N 4 nanosheets , and g-C 3 N 4 /Ag-TiO 2 nanocomposites was examined by assessing their ability to degrade rhodamine B (RhB) upon illumination by energy-saving visible LED light.
2.2. Synthesis 2.2.1. Preparation of g-C 3 N 4 Samples g-C 3 N 4 nanosheets were prepared by one-step calcination of a mixture of urea and thiourea in a muffle furnace under an air atmosphere. Typically, specific amounts of urea and thiourea were carefully weighed and placed into a 50 mL ceramic crucible with a lid, which was then wrapped with aluminum foil paper and subjected to calcination at 600 • C for 3 h. The resulting product was cooled to room temperature and carefully ground into a fine powder.

Preparation of Ag-TiO 2 Nanoparticles
Ag-doped TiO 2 (Ag-TiO 2 ) nanoparticles were prepared using a sol-gel method. First, solution A was obtained by dissolving 22 mL of titanium isopropoxide (TTIP) in 150 mL of ethanol, followed by constant stirring for 45 min. In solution B, 3 mol% of silver nitrate was dissolved in 40 mL of distilled water with the addition of 5 mL of glacial acetic acid. Solution B was then slowly dripped into solution A and vigorously stirred. The resulting milky sol was constantly stirred for 2 h while carefully adjusting the pH value to 2-3 by adding 2 mL of nitric acid. Subsequently, the white precipitate obtained from the sol was allowed to settle for 4 days at room temperature, after which the solid gel was collected and washed several times with distilled water by centrifugation at 4000 rpm for 10 min. The washed samples were dried at 80 • C for 16 h in a vacuum oven. The resulting products were ground and calcined at 400 • C for 4 h. Finally, the samples were ground again and used for characterization. Similarly, pristine TiO 2 nanoparticles were synthesized under the same experimental conditions without the addition of AgNO 3 as a precursor of Ag.

Fabrication of g-C 3 N 4 /Ag-TiO 2 Nanocomposite
A facile hydrothermal method followed by calcination was used to fabricate a heterostructured g-C 3 N 4 /Ag-TiO 2 nanocomposite with a weight ratio of 3:1 of fabricated g-C 3 N 4 nanosheets and 3% Ag-TiO 2 nanoparticles, respectively. The process involves dissolving 0.3 g of g-C 3 N 4 nanosheets in 100 mL of distilled water by ultrasonication for 1 h and then adding 0.1 g of 3% Ag-TiO 2 nanoparticles to the solution, followed by vigorous stirring for 2 h. In addition, a small amount of hydrofluoric acid was added continuously during stirring to control nanocrystal formation. The resulting solution was transferred to a 100 mL stainless-steel autoclave and maintained at 180 • C for 6 h. Following hydrothermal treatment, the mixture was cooled to room temperature and subsequently washed several times with distilled water. Finally, the resulting sample was dried at 80 • C for 15 h and then ground into an ultrafine powder.

Characterization
The crystal structures of the as-synthesized samples were analyzed using X-ray powder diffraction (PanAlyticals, Almelo, The Netherlands) with Cu-Kα radiation (λ = 1.5406 Å) operating at 40 kV and 15 mA. The diffraction patterns were measured over a range of 10-70 • , with a step width of 0.01 • and a scanning rate of 10 • /min. The functional groups and chemical structures of the samples were analyzed using FTIR spectroscopy (IR Tracer-100, Shimadzu, Kyoto, Japan) within the wavenumber range of 600-3600 cm −1 . The bandgap energies of the samples were analyzed using UV/vis diffuse reflectance spectroscopy (UV-2600, Shimadzu, Kyoto, Japan) with barium sulfate as the background substance within the wavelength range of 300-700 nm. The separation of photoinduced electron-hole pairs in the as-prepared samples was examined using PL spectrophotometer (RF-6000, Shimadzu, Kyoto, Japan) with an excitation wavelength of 360 nm. Field-emission scanning electron microscopy (FE-SEM, MAIA3 XMH, TESCAN BRONO s.r.o, Brono, Czech) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL, Tokyo, Japan) were used to investigate the morphology of the samples at an accelerating voltage of 200 kV. The N 2 adsorption-desorption isotherms and Brunauer-Emmett-Teller specific surface area (S BET ) analyses were performed using a Micromeritics ASAP 3000 (Micromeritics, Norcross, GA, USA) equipped with a nitrogen adsorption device. Additionally, the Barrett-Joyner-Halenda (BJH) method was used to determine the pore size distribution and volume curves of the as-synthesized samples.

Photocatalytic Activity Testing
The photocatalytic activities of as-prepared photocatalysts were determined by assessing their ability to degrade rhodamine B (RhB) using a 50 W crystal white LED lamp (5000 lm, 6500 k, 220-240 V) as a visible light source (Phillips, Kolkata, India). A photolysis experiment was conducted on an aqueous RhB solution under visible LED light illumination for 180 min. Adsorption and photocatalytic experiments were performed by dispersing 50 mg of each photocatalyst in 100 mL of a 10 mg/L RhB aqueous solution. A dark adsorption experiment was conducted for 60 min to establish the adsorption-desorption equilibrium between RhB and the photocatalysts. Throughout the experiments, vigorous magnetic stirring was applied. At specific time intervals during each experiment, 3 mL aliquots were withdrawn from the solution and subjected to centrifugation to separate the photocatalyst. The absorbance of RhB was measured at its maximum absorption wavelength (λ = 554 nm) using a UV/vis spectrophotometer (2450, Shimadzu, Kyoto, Japan). The rate of RhB dye degradation (η) in aqueous solution was calculated using the following equation [46,47]: where A 0 and A refer to the initial absorbance and absorbance at a specific irradiation time t, respectively. A pseudo-first-order kinetic model was utilized to determine the apparent degradation rate constant (k app ) of RhB dye over g-C 3 N 4 /Ag-TiO 2 composites [11,47]: where k app (min −1 ) is the rate constant and t (min) is the specific time interval of irradiation.

XRD Analysis
The crystal structures of the as-prepared samples, including pristine TiO 2 , Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4 /Ag-TiO 2 , were investigated using X-ray powder diffraction (XRD), and the results are shown in Figure 1. The XRD patterns of g-C 3 N 4 revealed the presence of two characteristic peaks at around 13.1 • (100) and 27.2 • (002), which correspond to in-plane repeating tri-s-triazine units and interplanar stacking of conjugated aromatic units, respectively [6,48]. The absence of a characteristic Ag peak in the composites, as confirmed by the XRD spectra, can be attributed to the low amount of Ag or its high dispersion [38,41,42]. sample was primarily composed of the anatase phase, with a minor brookite phase at 2θ of 30.68 • , corresponding to the (121) crystal plane, which indicated consistency with previous findings [49]. The XRD patterns of the g-C 3 N 4 /Ag-TiO 2 nanocomposite contained diffraction peaks arising from both Ag-TiO 2 and g-C 3 N 4 , thereby confirming the successful formation of the composite. Moreover, a higher g-C 3 N 4 content was found to be capable of intensifying the recognizable peak at approximately 27.6 • , resulting in a minor peak position shift in the composite [11]. Both anatase and rutile TiO 2 diffraction peaks were observed for the g-C 3 N 4 /Ag-TiO 2 composite [42,50].  The average crystallite size (D) of the as-synthesized photocatalysts was estimated using the Scherrer equation [21]: where K is a constant (0.94), λ is the X-ray wavelength (0.15406 nm), β is the full width at half maximum (FWHM), and θ is the diffraction angle. As calculated from the Scherrer equation, the average crystallite sizes of the as-synthesized photocatalysts, including pure TiO 2 , Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4 /Ag-TiO 2 , were approximately 5.2, 9.1, 4.3, and 7.6 nm, respectively.

UV/vis DRS Analysis
The light absorption properties and bandgap energies of all synthesized samples, including TiO 2 , g-C 3 N 4 , Ag-TiO 2 , and g-C 3 N 4 /Ag-TiO 2 , were analyzed using UV/vis diffuse reflectance spectroscopy (DRS), as shown in Figure 2.
As shown in Figure 2b, the bandgap energies of the Ag-TiO 2 and g-C 3 N 4 /Ag-TiO 2 composites are narrower than those of the pristine TiO 2 and g-C 3 N 4 samples. The absorption edges of the g-C 3 N 4 /Ag-TiO 2 nanocomposite exhibited a considerable redshift toward higher wavelengths, indicating a significant improvement in the visible light absorption of the composites, as shown in Figure 2a [11,50]. This may be due to the surface plasmon resonance effect of Ag, which is deposited on the surface of the TiO 2 nanoparticles, and the formation of heterostructures in the composites [17]. The bandgap energy (E g ) of the as-synthesized samples was calculated using a modified Tauc equation with the Kubelka-Munk function: (F(R)hυ) is the Kubelka-Munk function, R is the reflectance, hυ is the photon energy, and A is a constant [11]. The bandgap energy can be estimated by determining the point at which the linear trendline on the plot of (F(R)hυ) 1/2 versus (hυ) intersects the hυ-axis [11,42]. The bandgap energies of TiO 2 , Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4/ Ag-TiO 2 photocatalysts were calculated to be 3.04, 2.89, 2.94, and 2.73 eV, respectively. Compared to pristine TiO 2 and g-C 3 N 4 , Ag-TiO 2 shows a slight red shift in the absorption edge [9]. Furthermore, coupling g-C 3 N 4 with Ag-TiO 2 shows a significant red shift compared to TiO 2 , Ag-TiO 2 , and g-C 3 N 4 , which allows for enhanced visible-light absorption [48]. Therefore, the synergetic effect of coupling g-C 3 N 4 and loading Ag on TiO 2 ameliorated the separation and transportation of photogenerated charge carriers, leading to an overall increase in photocatalytic performance [9,51].

PL Analysis
Photoluminescence (PL) analysis was used to study the separation of photoinduced electron-hole pairs in the as-synthesized samples [43]. Figure 3 shows the PL spectra of pristine TiO 2 , Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4 /Ag-TiO 2 samples obtained at an excitation wavelength of 360 nm. Both g-C 3 N 4 and g-C 3 N 4 /Ag-TiO 2 displayed main emission peaks at approximately 440 nm, as shown in Figure 3. A higher PL intensity was observed for g-C 3 N 4 compared to g-C 3 N 4 /Ag-TiO 2 , indicating faster recombination of photoinduced electron-hole pairs in g-C 3 N 4 [36]. The PL spectra of pristine TiO 2 and Ag-TiO 2 nanoparticles did not exhibit any significant emission peaks, indicating that they had a lower rate of recombination of photogenerated electron-hole pairs [50]. The reduced PL emission peak of the g-C 3 N 4 /Ag-TiO 2 composite compared to g-C 3 N 4 can be attributed to a lower rate of recombination of photogenerated electron-hole pairs, which may be due to the formation of a heterostructure between g-C 3 N 4 and Ag-TiO 2 [42,43]. Figure 4 shows the results of the FTIR analysis conducted to identify the functional groups and chemical structures of all-prepared photocatalysts.

FTIR Analysis
The stretching modes of C-N and C=N heterocycles were observed at 1200-1740 cm −1 in the g-C 3 N 4 samples [6,52]. In the g-C 3 N 4 samples, peaks observed at around 1458, 1560, and 1630 cm −1 can be attributed to C=N heterocycles, and the stretching mode of C=O was detected at 1740 cm −1 . Additionally, the FTIR peaks at 1230, 1319, and 1400 cm −1 represent the aromatic C-N stretching mode, whereas the peaks at 805 and 885 cm −1 indicate the breathing mode of the tri-s-triazine ring units. The presence of broad bands from 3040-3300 cm −1 shows N-H and O-H stretching modes, which can be attributed to absorbed H 2 O molecules [6,21,47,53].

CN heterocycles
Tri-s-triazine units  The broad peaks appearing at 3300-3000 cm −1 are due to the stretching vibrations of the O-H group on the TiO 2 surface [19,54]. The peak located around 1626 cm −1 is due to bending vibrations resulting from chemically adsorbed H 2 O molecules [18]. The stretching vibrations of Ti-O and Ti-O-Ti exhibited by the TiO 2 nanoparticles were observed at around 850 cm −1 [11,19,55]. The peaks at 1216, 1366, 1438, and 1738 cm −1 correspond to the carbonyl (C=O) mode. The peak observed at 1180 cm −1 corresponds to the stretching vibrations of the Ti-OH molecule [13]. In the g-C 3 N 4 /Ag-TiO 2 sample, characteristic peaks attributed to both TiO 2 and g-C 3 N 4 were observed, confirming the presence of both materials in the composite sample [11,21]. Figure 5 shows the FE-SEM and HR-TEM morphologies of g-C 3 N 4, Ag-TiO 2 , and g-C 3 N 4 /Ag-TiO 2 samples.  The SEM micrograph of pristine TiO 2 nanoparticles exhibits irregularly shaped agglomerates, as shown in Figure 5a. The SEM and TEM images of Ag-doped TiO 2 nanoparticles are shown in Figure 5b,e, respectively. In Figure 5b, the SEM image depicts the agglomeration-induced formation of a cluster of Ag-TiO 2 nanoparticles and the homogeneous distribution of spherical Ag-TiO 2 nanoparticles [56,57]. The presence of small Ag 0 islands on the surface of Ag-doped TiO 2 nanoparticles is clearly observed in the TEM image, as shown in Figure 5e [58]. The SEM image shows significant adhesion between the Ag-TiO 2 nanoparticles and the g-C 3 N 4 nanosheet, indicating a strong interface between the two constituents of the composite, as shown in Figure 5c. Notably, the presence of g-C 3 N 4 improves the dispersion of TiO 2 nanoparticles by preventing their agglomeration [11].

Morphological Analysis
Based on the TEM image shown in Figure 5d, the g-C 3 N 4 nanosheet exhibited a transparent appearance, which can be attributed to its thin and two-dimensional structure. These results also indicate that g-C 3 N 4 has a fluffy structure [59].
The TEM image in Figure 5f depicts a strong interfacial region between g-C 3 N 4 and Ag-TiO 2 . As can be seen in Figure 5f, the layered structure with the lightest color was identified as g-C 3 N 4 , whereas the stacked structure comprising small particles was inferred to be TiO 2 . The black particles situated between TiO 2 and g-C 3 N 4 were identified as Ag nanoparticles [38]. The lattice fringe spacing of TiO 2 was 0.377 nm, which corresponds to the (101) crystal plane of TiO 2 [42]. Therefore, it can be concluded that the TEM images of the g-C 3 N 4 /Ag-TiO 2 nanocomposite represent the microstructure derived from its constituents, such as TiO 2 , g-C 3 N 4 , and Ag nanoparticles, thereby demonstrating the successful formation of the composite. Moreover, the results indicated that the agglomerated Ag-TiO 2 nanoparticles were well dispersed on the porous architecture of g-C 3 N 4 . In addition, creating a close interfacial contact between g-C 3 N 4 and Ag-TiO 2 heterostructures may effectively promote the transfer of photoinduced electrons. This could potentially suppress the rate of charge-carrier recombination and enhance the overall photocatalytic activity [41,42].  Based on the N 2 adsorption-desorption isotherms, all synthesized photocatalysts exhibited typical IV isotherms of mesoporous materials with H1, H2, and H3 hysteresis loops for g-C 3 N 4 , Ag-TiO 2 , and g-C 3 N 4 /Ag-TiO 2 , respectively, as shown in Figure 6a [17,21,43]. The specific surface area of Ag-TiO 2 nanoparticles increased slightly because of the incorporation of Ag nanoparticles. However, the pore volume of Ag-TiO 2 is not as high as its BET surface area because the Ag nanoparticles tend to block the pores in TiO 2 . Moreover, the surface area of the composite slightly decreased with the coupling of Ag-TiO 2 to g-C 3 N 4 , potentially owing to the covering and blocking of the surface-active sites of Ag-TiO 2 by g-C 3 N 4 [21]. However, the excellent photocatalytic degradation efficiency of the g-C 3 N 4 /Ag-TiO 2 nanocomposite might not be underestimated owing to the small specific surface area. In addition, the excellent photocatalytic performance of the composite is a consequence of a combination of factors such as the extended visible light absorption range, enhanced crystallite size, and reduced recombination of charge carriers owing to its narrowed bandgap energy [15]. Previous studies have reported that g-C 3 N 4 /Ag-TiO 2 nanocomposites exhibit excellent photocatalytic performance despite having a smaller surface area than their constituent components [60]. Figure 6b shows the pore size distribution of the as-synthesized samples obtained using the Barrett-Joyner-Halenda method. The average pore sizes of the Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4 /Ag-TiO 2 samples were 6.5, 25.3, and 10.5 nm, with the corresponding total pore volumes of 0.15, 0.52, and 0.06 cm 3 ·g −1 , respectively. The g-C 3 N 4 nanosheet showed the highest values for both pore size and total pore volume, which are essential factors in enhancing photocatalytic activity. The average crystallite size, energy bandgap, specific BET surface area (S BET ), average pore size, and pore volume of the as-synthesized samples, including Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4 /Ag-TiO 2 nanocomposites, are summarized in Table 1. Table 1. Crystallite size, energy bandgap (E g ), specific surface area (S BET ), pore size, and pore volume of the as-synthesized samples.   The photolysis experiments were conducted in the absence of as-prepared photocatalysts, and almost no degradation of RhB dye was observed. The adsorption experiments were performed using the as-synthesized samples in the dark for 60 min. The g-C 3 N 4 /Ag-TiO 2 nanocomposite exhibited an adsorption rate of 10%. In contrast, TiO 2 exhibited the lowest adsorption rate of only 1.15%, whereas Ag-TiO 2 and g-C 3 N 4 had adsorption rates of 7% and 3.5%, respectively. The photocatalytic activities of all the prepared photocatalysts were evaluated by studying the photocatalytic decomposition efficiency of RhB dye under visible LED light irradiation. After 180 min of irradiation with visible LED light, g-C 3 N 4 and g-C 3 N 4 /Ag-TiO 2 photocatalysts exhibited remarkably higher photodegradation efficiencies of 94.83% and 98.04%, respectively, whereas pure TiO 2 showed the lowest photodegradation efficiency of 25.9%. The binary Ag-TiO 2 nanoparticles exhibited an intermediate photodegradation efficiency of 56.73%. The rate of degradation of binary Ag-TiO 2 photocatalysts is higher than that of pristine TiO 2 but lower than that of g-C 3 N 4 and g-C 3 N 4 /Ag-TiO 2 nanocomposites [48].

Evaluation of Photocatalytic Activity
The enhanced photocatalytic activity of the g-C 3 N 4 /Ag-TiO 2 composite was due to the synergetic effect of the localized surface plasmon resonance (LSPR) impact of Ag and its role as an electron-conduction bridge, as well as the existence of an excellent heterostructure between Ag-TiO 2 and g-C 3 N 4 . These factors ameliorate the charge carrier separation efficiency and inhibit electron-hole pair recombination, which boosts the overall photocatalytic performance [38]. In terms of photogenerated charge separation, g-C 3 N 4 has a more negative conduction band potential than that of TiO 2 . Meanwhile, the Ag nanoparticles serve as charge separation centers for photogenerated electrons from the conduction band of TiO 2 owing to their lower Fermi level energy. Consequently, the photogenerated electrons and holes can be easily transported to the adjacent semiconductor surface, thus suppressing the rate of recombination of photoinduced electron-hole pairs, leading to improvements in photocatalytic efficiency [38,42]. Figure 7b shows the linear pseudo-first-order kinetics of the RhB dye degradation. The apparent reaction rate constants (k app ) of TiO 2 , Ag-TiO 2 , g-C 3 N 4 , and g-C 3 N 4 /Ag-TiO 2 were directly calculated by the equation ln(C/C 0 ) = k app t, as shown in Figure 7b. The apparent rate constant (k app ) of TiO 2 nanoparticles is 1.66 × 10 −3 min −1 because the catalytic activity of TiO 2 is lower under visible light. The k app of Ag-TiO 2 is 4.65 × 10 −3 min −1 , which is mainly due to the plasmon resonance effect of the Ag nanoparticles on the surface of the TiO 2 nanoparticles. The k app of g-C 3 N 4 is 16.46 × 10 −3 min −1 and that of the g-C 3 N 4 /Ag-TiO 2 nanocomposite is increased to 21.84 × 10 −3 min −1 . Therefore, the g-C 3 N 4 /Ag-TiO 2 nanocomposite exhibited a reaction rate constant 13.16, 4.7, and 1.33 times higher than that of TiO 2 , Ag-TiO 2, and g-C 3 N 4 , respectively. The synergistic effect of the heterostructure comprising g-C 3 N 4 nanosheets, TiO 2 , and Ag nanoparticles leads to enhanced photodegradation efficiency by effectively accelerating the transfer of photogenerated electrons and inhibiting the fast recombination of electron-hole pairs [17]. The photocatalytic degradation efficiency of g-C 3 N 4 /Ag-TiO 2 nanocomposites was compared with that of various previous studies in Table 2. We found that the g-C 3 N 4 /Ag-TiO 2 nanocomposite fabricated in this study demonstrated excellent photocatalytic activity compared to those in previous works.

Mechanism of Photocatalytic Degradation
Based on the results and discussions presented, including XRD, UV/vis DRS, and PL analyses, as well as previous reports [11,15,17,38,41], a plausible photodegradation mechanism for ternary g-C 3 N 4 /Ag-TiO 2 photocatalysts is proposed in Figure 8. The conduction band (E CB ) and valence band (E VB ) potentials relative to the normal hydrogen electrode (NHE) of the samples were determined using the following equations: where χ is the absolute electronegativity of the semiconductor; TiO 2 = 5.81 eV and g-C 3 N 4 = 4.73 eV. The energy of free electrons on the hydrogen scale is denoted as E e and has a value of 4.5 eV, and E g is the bandgap energy [42,43]. From UV/vis DRS analysis results, the valence band and conduction band potentials of g-C 3 N 4 were 1.7 eV and −1.24 eV, while those of TiO 2 were 2.83 eV and −0.21 eV, respectively.   Figure 8 shows that under visible LED light irradiation, electrons were excited from the valence band to the conduction band (CB) of TiO 2 and g-C 3 N 4 . Electrons could easily migrate from g-C 3 N 4 to TiO 2 because of the more negative conduction band potential of g-C 3 N 4 (E VB = −1.24 eV) than that of TiO 2 (E VB = −0.21 eV). These electrons combine with the adsorbed O 2 on the TiO 2 surface to form • O 2 − , which is then used for the decolorization of RhB. Similarly, it appears that g-C 3 N 4 (+1.7 V vs. NHE) has a lower valence band potential than H 2 O/ • OH (+2.38 V vs. NHE), indicating that holes are incapable of oxidizing H 2 O into • OH radical species; however, they can directly participate in RhB oxidation. Therefore, holes and • O 2 − radicals may play a major role in the decolorization of RhB into harmless byproducts such as H 2 O and CO 2 [17,42,43,61].
Ag deposited on TiO 2 in g-C 3 N 4 /Ag-TiO 2 nanocomposites acts as a bridge for electron conduction, resulting in improved separation of electron-hole pairs on g-C 3 N 4 through the generation of a Schottky barrier between Ag nanoparticles and TiO 2 [17,38]. In addition, due to the surface plasma resonance effect, Ag nanoparticles in heterostructured g-C 3 N 4 /Ag-TiO 2 nanocomposites can be photoexcited, producing electrons that migrate to the conduction band of TiO 2 [48]. The transfer of excess electrons from the conduction band of TiO 2 to Ag nanoparticles in heterostructured g-C 3 N 4 /Ag-TiO 2 nanocomposites reduces the recombination of photoinduced charge carriers [15]. The incorporation of Ag nanoparticles, in addition to the excellent heterostructure formation between TiO 2 and g-C 3 N 4, results in excellent photocatalytic degradation efficiency of RhB [17,38]. Moreover, the close interfacial connections between Ag-TiO 2 and g-C 3 N 4 allow for efficient electron migration and spatial separation of electrons and holes, retarding their recombination rate and thereby improving photoactivity [60].

Conclusions
A simple hydrothermal technique followed by calcination was used to fabricate g-C 3 N 4 /Ag-TiO 2 composites with high photocatalytic performance for RhB dye degradation under visible light irradiation. The heterostructured g-C 3 N 4 /Ag-TiO 2 composite was synthesized under optimal conditions at 180 • C for 6 h with a 3:1 weight ratio of g-C 3 N 4 to Ag-TiO 2 . Various analytical tools were employed to characterize the crystal structures, morphologies, microstructures, chemical structures, and optical and physicochemical properties of the as-synthesized samples. The experimental results demonstrate that the heterostructured g-C 3 N 4 /Ag-TiO 2 composite is outstanding for the decomposition of RhB dye under visible LED light irradiation and exhibits a degradation efficiency of 98.04%. The high photocatalytic activity observed for the g-C 3 N 4 /Ag-TiO 2 nanocomposite may be due to the synergetic effect of the LSPR effect of Ag and its role as an electronconduction bridge, as well as the formation of a strong heterostructure between Ag-TiO 2 and g-C 3 N 4 . These factors significantly enhanced the efficiency of photogenerated charge carrier separation and transfer, improved visible light absorption, increased crystallite size, and inhibited charge carrier recombination, thereby improving the overall photocatalytic performance. Therefore, the present study shows that g-C 3 N 4 /Ag-TiO 2 nanocomposites possess enormous potential for the effective elimination of hazardous organic pollutants in wastewater.

Data Availability Statement:
The data that support the findings of this study are available upon reasonable request from the corresponding author.